Kevin Garnett has committed to returning to the Celtics, agreeing to a three-year contract believed to be worth around $34 million, according to an NBA source. Garnett, an unrestricted free agent, informed the Celtics before Sunday’s start of free agency negotiations, which should help the team accelerate talks with several players.

With Garnett, the Celtics will have five players under contract for the 2012-13 season, including team captain Paul Pierce and Rajon Rondo. Free agent signings don’t become official until July 11; until then, the Celtics will decide whether to bring back unrestricted free agents Ray Allen, Brandon Bass, Jeff Green, and Mickael Pietrus, among others.

After considering retirement, Garnett decided to remain a Celtic despite the fact that he will receive an annual salary greatly reduced from the $21 million he received last season.

Garnett has found a level of comfort with the Celtics, developing a strong bond with teammates and coach Doc Rivers. Garnett, who turned 36 last month, has earned $291 million over 17 NBA seasons, $106
million in five years with the Celtics.

Garnett is also taking in big sponsorship money from Anta Sports Products Ltd., a Chinese apparel company. Though that relationship might not be dependent on Garnett being an active player, it is doubtless better for Garnett if he is playing in the NBA.

Garnett has played 1,380 games, including playoffs, and totaled 45,779 regular-season minutes, 10th all time.
But Rivers has limited Garnett’s minutes to around 30-31 per game the last four years.

Rivers was also able to convince Garnett of the benefits of moving from power forward to center during the second half of last season and Garnett appeared rejuvenated. After the Celtics got off to a 15-17 start, Garnett went to center in place of the injured Jermaine O’Neal and the Celtics’ offensive dynamic changed. Garnett became more willing to establish himself on the post and take perimeter jumpers, and the Celtics ended the regular season with a 39-27 record and the Atlantic Division title.

Garnett has been credited with changing the culture of the Celtics since joining the team, along with Allen, for the 2007-08 championship season, adding intangibles with his defense and unselfish team approach to offense.

Garnett has averaged 65.6 games per season with the Celtics, limited to 57 in the 2008-09 season by a knee injury. He missed the postseason and underwent surgery for a damaged popliteus tendon and bone spurs. Garnett did not fully recover from the procedure until last season, according to Pierce.

Last season, Garnett averaged 15.8 points and 8.2 rebounds per game. He was even stronger in the playoffs, averaging 19.2 points and 10.3 rebounds as the Celtics advanced to the Eastern Conference finals.

Though Garnett thrived at center, the Celtics will attempt to find another big man during the offseason, opening the option for Garnett to return to power forward, at least on a part-time basis.